Most polymeric materials are not conductive. Over the past fifteen years, some organic polymers have been developed as conducting materials and termed synthetic metals. In general, they contain an extendable .pi.-electron resonance system, i.e. a conjugation through the polymer chain. Therefore, the relationship between the chemical structure and the conductivity in the repeating unit of the polymer is of significance.
Polyaniline (PAN), the first conducting polymer to be investigated, was obtained from a sulfuric acid solution via anodic oxidation of aniline by H. Letheby in 1862. However, it was polyacetylene that triggered the study of conducting polymers, which are now fully understood. In the past decade, conducting polymers have been used in many fields, such as batteries, displays, optics, the aeronautical industry, and defense industry. Nonetheless, the stability caused by the physical properties as well as certain processing problems have not been thoroughly solved.
Conductivity is mainly determined by the product of two factors: the number of the carriers, i.e. electrons or holes; and the mobility of the carriers. The conductivity of most conducting polymers is similar in range to that of inorganic semi-conductors which have very few carriers (10.sup.16 -10.sup.18 /cc) but high mobility (10.sup.2 -10.sup.5 cm.sup.2 /volt-sec) due to their high crystallinity, good purity, and few defects. Most conducting polymers are amorphous or partially crystalline, with carriers number of about 10.sup.21 -10.sup.23 /cc that are about 10.sup.4 -10.sup.5 times larger than that of the inorganic semi-conductors, and low mobilities in the range of 10.sup.-4 -10.sup.-5 cm/volt-sec [M.G. Kanatzidis, Chemical & Engineering News, 68(49), (1990) 36]. Therefore, improvement in mobility is an essential requirement for enhancing conductivity of the conducting polymers. A synthetic method for producing materials with high crystallinity, good alignment, and few defects has not been found so far. Methods for modifying the main chain alignment of conducting polymers include:
(1) Polymerizing in the liquid crystalline solution to obtain high orientation of the product; PA1 (2) Realigning polymers in a magnetic field; PA1 (3) Stretching polymer film or fiber; PA1 (4) Blending with precursors which are processable and soluble, and followed by heat treatment; and PA1 (5) Encapsulating organic monomers to the holes of the regular inorganic lattices and followed by polymerization.
Among the above methods, encapsulation is employed for versatile composite materials. The idea is to offer a regular reaction room for the polymer and force its main chain into better alignment, as opposed to the tangled polymers produced by conventional synthesis. A decrease in the number of defects in the polymers' main chains is obtained from such a restricted environment thereof.
For example, polypyrrole and polythiophene fibers are made respectively in the holes of commercialized Nucleopore and Anopore alumina filtration membranes [Z. Cai and C.R. Martin, J. Am. Chem. Soc., 111 (1989) 4138]. Electrochemical synthesis is also utilized in the production of whiskers of the above polymers [W. Cahalene and M. Mortimer, Synth. Met., 43 (1991) 3079]. Their conductivities fall in the range of 3,000-7,500 S/cm. FeOC1 can be encapsulated by either pyrrole or aniline, and polymers formed in the layers. These resulted inclusion compounds both display 1 S/cm conductivity [M.G. Kanatzidis, et al., J. Am. Chem. Soc., 109 (1987) 3797; C.G. Wu, et al., Abs. of Papers of the Am. chem. Soc., 199 (1990) 354] at room temperature as well as the thermotropic metal behavior of PAN/FeOC1 at 220.degree. K. Some conducting polymers can be encapsulated into the layers of V.sub.2 O.sub.5, allowing the composite to be processed into anisotropic film. By controling the polymer content, n-type conductors can be converted to p-type ones, in addition to displaying good weather-resistance [M.G. Kanatizidis, et al., J. Am. Chem. Soc., 111 (1989) 4139]. Zeolite is another host candidate for the inclusion compound [T. Bein and P. Enzel, Mol. Cryst. Liq. Cryst., 181 (1990) 315]. The hole size of zeolite is exactly the magnitude of one polymer main chain, thus making the polymer non-interactive with the others, as well as providing insulation properties. In the future, the individual conjugated polymers or oligomers produced by this method may possibly be used as molecular wires in molecular electronics.
PAN is generally made of at least 1,000 aniline monomers through electrochemical or chemical-oxidizing polymerization. There are four oxidation forms of PAN shown below: ##STR1## with conductivities of about 10.sup.-11 to 5 S/cm. PAN has different color and electricity depending upon their structures. Emeraldine salt (PAN-2S) is conductive [A.G. MacDiarmid, et al., in "Conducting Polymers Special Applications" (eds. L. Alcacer) D. Reidel Publishing Co. Holland, 1987, p. 105]. It becomes emeraldine base (PAN-2A) with no conductivity after base treatment. PAN-2A will be highly conductive without partial oxidation or reduction occured in the structure after simple protonation of the N-atoms on the main chain imine groups. The electron numbers of the polymer structure do not change after proton-blending. Therefore, the increased conductivity is related to the acidity of the solution. The effects of blending on the conductivity of PAN have been studied by several groups. Although the structure of PAN has not been fully determined so far, a linear model is generally recognized as follows: ##STR2## PAN is generally amorphous. But either PAN-2S or PAN-2A is considered as partially amorphous by Y. Cao, et al. [D. Vachon, et al., Synth. Met., 18 (1987) 297; S.D. Philips, et al., Phys. Rev. B. 39 ( 1989 ) 702 ].
Most conducting polymers are not dissolved in general organic solvents. This creates the problems with determining molecular weight. There are two number average molecular weight (M.sub.n) distributions for the insulating type PAN which is soluble in N-methylpyrrolidinone (NMP). The lower part of M.sub.n distribution is at about 4,800 and the higher part is in the range of 200,000-350,000 [X. Tang, et al., Rapid Commun., 9 (1988) 829; A.G. MacDiarmid, et al., Polymer Eng. and Sci., 31 (1991) 147]. These M.sub.n distributions result from a two-step polymerization. Pernigraniline is thus produced as an oxidated state of PAN by free radical/cation polymerization. The lower M.sub.n part is produced by oxidation between pernigraniline and aniline. The conducting form of PAN, i.e. PAN-2S, dissolves in concentrated sulfuric acid (97%) but not in NMP solution, which has molecular weight of about 40,000 estimated by its viscosity [A. Andreatta, et al., Synth. Met., 41 (1991) 2305].
The properties of PAN are deeply affected by the conditions of polymerization, such as oxidant, ratio of oxidant and monomer, acidic medium, reaction temperature, and protonation level.
The following drawings provide a brief introduction to the inclusion chemistry which is a background of the present invention.